NF-kappaB/Rel transcription factors are a family of five proteins which function as dimer complexes in the regulation of genes involved in innate and specific immunity. This study will examine how these transcription factors regulate in vivo immune responses. Our experimental approach will utilize gene-targeted mice for individual family members in combination with two unique experimental approaches. The first approach uses a histochemical marker that directly monitors in vivo NF-kappaB/Rel activation. This marker was created by targeted introduction of a lacZ reporter gene into the I-kappa-B-alpha locus. The second approach uses retroviral vectors tagged with a green fluorescent protein to reconstitute primary B-cells with specific forms of NF-kappaB/Rel proteins. Retroviral reconstitution will be used to determine how individual complexes regulate class switching and immunoglobulin secretion in B-cells. Defining the natural physiological roles of these factors will directly contribute to understanding why certain members are proto-oncogenes, and how their disregulation can lead to oncogenic activation in human cancer. Determining how these factors regulate immune responses will also produce a better understanding of the in vivo action of clinically relevant drugs, such as aspirin and glucocorticoids, that have been identified as inhibitors of this transcription factor family. Specific Aim 1 will determine when and where NF-kappaB/Rel complexes are activated in vivo during the natural history of an immune response to a T-cell dependent antigen. Specific Aim 2 will examine in vivo how the absence of individual NF-kappaB/Rel factors disrupts effective immune responses and determine the cellular basis for defective responses. Aim 3 will define roles of specific NF-kappaB/Rel complexes in primary B-cells in regulating immunoglobulin secretion and class switching.